1. Field of the Invention
The subject invention relates to systems for clutches and brakes, and more particularly to an improved clutch and brake assembly. Use of self-energizing clutches has been widely used and well understood in the art.
2. Background of the Related Art
Clutches are used generally to control the transmission of torque between rotating machine elements. Positive clutches, when engaged, lock the elements together to rotate as one. Such clutches may be xe2x80x9cself-energizingxe2x80x9d to the extent that the force of engagement increases as the torque between the shafts increases.
It is known to those skilled in the art to produce electrical, self-energizing, positive clutches utilizing a helical spring attached at one end to one rotating machine element, and electromagnetically attracted to the second rotating machine element by an electromagnetic coil when the clutch is to be activated. Twisting, of the helical spring caused by relative motion of the two rotating machine elements causes the spring to tighten about an annular wedge of frictional material to press it against opposed faces of the two machine elements bringing them into frictional linking. The helical spring and the wedge shape of the frictional material define a mechanical advantage that may allow a relatively low force of attraction between the electromagnet and the helical band to nevertheless provide a high torque coupling of the two machine elements in a desirable manner. One example of such a clutch is found in U.S. Pat. No. 3,149,706, and hereby incorporated by reference.
Often it is desirable, when the clutch is disengaged, to lock the driven machine element in place to prevent free-wheeling. For this purpose, an electric brake may be attached to the driven machine element. However, there are problems associated with this solution. For example, circuitry must be provided to energize the brake when the electric clutch is de-energized. Actuation of the brake while the clutch is engaged causes excessive wear. Further, the use of an electric brake increases the cost, size and complexity of the total drive mechanism. Therefore, there is a need for an improved clutch and brake assembly which has a minimum of components, high life cycle and aids in assuring adequate safety.
The preferred embodiment provides a combination electrical clutch and brake sharing many components to reduce bulk and expense. Generally, the invention modifies the helical spring clutch mechanism so that when the spring is untorsioned, it expands radially outward against a brake sleeve restraining the driven machine element from free rotation. When under torsion, the helical band compresses a friction material to interconnect the driven machine element with the driving machine element while simultaneously releasing itself from the brake sleeve.
Specifically, the preferred embodiment provides an electric clutch and brake having a helical spring coaxial with an axis of rotation, the helical spring having a first and second end. A first machine element rotatable about the axis of rotation is attached to a first end of the helical spring. The first machine element has the first friction surface disposed on a first radial side of the helical spring. A second machine element rotatable about the axis of rotation has a second friction surface disposed on the first radial side of the helical spring and further has a pole face adjacent to the second end of the helical spring. An electromagnetic coil is positioned adjacent to the pole face so as to pull the second end of the helical spring into contact with the pole face when electrical current is passed through the electromagnetic coil. A retention plate having a third frictional surface is disposed on a second radial side of the helical spring opposite the first radial side of the helical spring. The helical spring causes frictional linking between the third frictional surface and the first machine element in a relaxed state when no current flows through the electromagnetic coil. The helical spring causes a frictional linking between the first and second frictional surfaces in a torsion state when current flows through the electromagnet coil.
Thus, it is one object to provide a simple combination clutch and brake mechanism. The same helical spring providing a frictional linking between the first and second machine elements provides a braking action to the first machine element when engaging this retention plate.
It is another object to provide a combination clutch and brake assembly in which clutching and braking action are mutually exclusive, reducing the possibility of unintentional clutch or brake wear. Because this same helical spring in alternate states provides both braking and clutching, the clutch and brake may not be simultaneously activated.
The first radial side of the helical spring may be the inside of the helical spring and the second radial side may be the outside of the helical spring.
It is another object, therefore, to provide an extremely compact brake clutch assembly. By placing the frictional surfaces in opposition radially, the total shaft length of the clutch brake assembly need not be increased over what would be required for the clutch alone.
The retention plate may be an annular cylinder coaxial with the axis of rotation having a radially inner surface adjacent to the outer radial surface of the helical spring. The frictional linking between the third frictional surface and the first machine element may be caused by a contacting of the outer radial surface of the helical spring with the inner surface of the annular cylinder.
Thus, it is another object to provide an extremely simple clutch and brake mechanism. In a preferred embodiment, by allowing the helical spring to directly contact the outer annular surface of the retention plate, the brake may be implemented with essentially no additional components over that required for the clutch alone.
One embodiment includes a device which comprises a cushioned clutch by reducing the friction. The soft action clutch can function integral with the brake assembly by sharing components. Another embodiment provides a brake pad assembly to more effectively brake a pulley. Further, an embodiment provides friction material between a clutch spring and a pole face to more effectively engage the two surfaces. Still further, an embodiment includes a spring-loaded bumper block to bias a friction disk to prevent premature wear and provide soft engagement. Another aspect of the invention is to provide a clutch which engages free of interference and still provides a reliable brake.
These and other unique features of the system disclosed herein will become more readily apparent from the following description, the accompanying drawings and the appended claims.